Recently, laser engraving or cutting of materials such as wood, plastic, paper, or the like has experienced enormous growth and public acceptance. Generally, laser engraving includes the use of a metallic mask perforated with a desired pattern which is placed in contact with a workpiece to be engraved. A powerful carbon dioxide laser beam, properly focused, is scanned or otherwise directed thereupon in an overlapping pattern to uniformly cover the perforations. Impingement of the focused laser upon the exposed material of the workpiece causes combustion or vaporization of the workpiece in areas where the mask perforations are situated. In contrast, where the beam strikes unperforated areas of the mask, the beam is either reflected (preferably) or absorbed, thus preventing engraving of the workpiece other than in the areas of the mask perforations.
This "contact mask" process is advantageous in that the contact between the mask and the workpiece substantially reduces optical aberrations introduced in the transferred image. Further, "reversed, inverted right-reading" issues in the workpiece images are eliminated. Finally, since the entire length of the image-projection beam path is no more than the thickness of the mask, the laser beam quality is unimportant.
While the "contact mask" process has proven satisfactory in many instances, several problems have been found in connection with its use. For example, the hot engraved-material vapor and soot tends to condense upon the outer surface of the mask, darkening it and reducing its reflectivity. A greater amount of beam energy is thus absorbed by the mask causing mask temperatures to elevate. As a result, the higher mask temperatures can leave burn marks on the paper and cause damage to the mask itself.
Further, any gap between the mask and the workpiece can cause vapor condensation therebetween, which often leaves an undesirable residue on the engraved workpiece. This residue is generally very tacky which often causes adherence between the workpiece and the mask. Moreover, if this gap permits mixing of oxygen-bearing air with the engraving vapor close to the workpiece, the vapor may ignite and damage or char the workpiece.
Another problem occurs when the laser engraving beam passes all the way through the workpiece, such as when engraving a sheet of paper. In this situation, the far side of the sheet is exposed to the same problems above-mentioned (i.e., vapor condensation, vapor burning, and ignition of the workpiece).
Attempts to ameliorate the various problems associated with contact-mask laser engraving have been numerous. One approach is to provide an additional mask, or a reflective or absorptive plate in contact with the backside of the engraved sheet. The thin flexible material, such as paper, is sandwiched between the mask and a backing plate, which subsequently, is passed under a laser beam (i.e., scanning). The additional backing, however, increases costs; and more importantly, is subject to the same problems of vapor condensation and overheating as that of the topside plate.
Another approach is to apply firm mechanical clamping between the mask and the workpiece to prevent vapor ignition or deposition on the workpiece surface. Typical of such arrangements are the devices found in U.S. Pat. No. 4,458,133. In this assembly, the mask is made of ferrous material having magnetic properties, while the workpiece, consisting of a sheet or stack of paper, is backed with a plate bearing one or more magnets. The uniform distribution of the magnets assure contact and an even distribution of clamping forces over the mask through the medium of magnetic attraction.
This magnetic configuration, however, is not particularly suitable for automated production. For example, assembly of the mask-paper "sandwich" is labor extensive. The sandwich, further, must be manually placed onto and removed from the conveyor, and the engraved sheets will have to be manually separated from one another after removal from the conveyor. Finally, the clamping force generated by the magnets is fairly limited. Accordingly, this type "contact mask" assembly is generally inappropriate for repetitive volume production.
Other clamping techniques have been employed which have improved the mask-to-workpiece contact. Screw-jacks and heavier masks substantially increase the contact force but are even more labor intensive when assembling the sandwich. Moreover, repetitive heating and cooling of the mask plate eventually causes warpage thereof to prematurely decrease the mask's lifespan.
More complex solutions involve positioning the mask remotely from and out of contact with the workpiece. This approach eliminates accumulation of condensed vapor or soot upon the mask, and avoids any mask-to-workpiece contact which may damage the workpiece. Single sheet workpieces, rather than multi-level sandwiches, can also be handled by this approach which is more suitable for automated manufacture. Lastly, the mask can be constantly cooled (via fans, blowers or the like) without mechanical interference with the engraving process. Examples of these laser engraving apparatus incorporating "non-contact mask" processes are disclosed in U.S. Pat. Nos. 4,156,124; 4,430,548; and 4,480,169.
While non-contact mask engraving has overcome some of the aforementioned problems, several new problems are introduced. For instance, one or more optical elements must be interposed between the workpiece and the mask to focus and more accurately control the laser beam path. This, of course, substantially increases the apparatus cost, as well as the cost of maintenance of such assemblies, which require constant recalibration. Each expensive optical element must be accurately positioned and aligned, and each is itself subject to damage from the accumulation of condensed engraving vapor and overheat caused thereby through reduced reflectivity. Further, the image engraved upon the paper becomes a distant projection of the mask image, introducing optical effects such as diffraction and depth-of-focus to the potential degradation of the engraved image.
Further still, while the mask has been removed from the effects of the engraving process, the surface of the workpiece itself is no longer protected by the mask from the condensation of engraving vapor nor from the sooting or ignition resultant from the burning of engraving vapor in close proximity thereto. Consequently, to protect the exposed workpiece sheet, the non-contact mask process requires the installation of a complex assembly of blowers, ducts and other exhaust extraction equipment for removing by-products.